The invention relates to an electrode separator for an electric cell and, in particular, an electrode separator capable of holding a larger quantity of electrolyte during operation of a lithium cell.
A lithium cell has an anode of manganese dioxide or a fluorocarbon and a cathode of the metal lithium. A separator holding an organic electrolyte such as propylene carbonate, .gamma.-butyrolactone, or dimethoxyethane is between the anode and the cathode. When the lithium cell is in use, lithium ions generated at the cathode move to the anode. The size of the anode gradually increases as the lithium ions accumulate and this subjects the separator to compression which discharges the electrolyte therefrom, leading to a reduction of the efficiency of the cell.
A compact mat of polypropylene filaments having diameters of 10.mu. or less has hitherto been used as the separator for the lithium cell. Quite sufficient electrolyte is held in the interstices between the fine filaments of this mat separator when it is not compressed. However, the mat separator of polypropylene filaments smaller than 10.mu. in diameter lacks sufficient stiffness to resist the compression caused by the expansion of the anode of the lithium cell to result in the discharge of the electrolyte and the lowering of the efficiency of the cell, as described.
To eliminate the above defect, the use of polypropylene filaments thicker than 10.mu. in diameter could be considered, but the increase in the diameter of the filaments increases the dimensions of the interstices between the filaments, thereby spoiling the essential role of the separator in preventing short circuits.